Detecting method and device for suppressing interference of low-frequency noise

ABSTRACT

The invention detects the difference between the maximum signal and the minimum signal at each of a plurality of cycles separately when a sine wave is received. All differences are summed for generating a single detected signal for suppressing the interference of low-frequency noise. No synchronization with the sine wave is necessary and the detection can start at any phase of the sine wave.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a signal retrieving module, and more particularly, to a signal retrieving module capable of filtering out high- and low-frequency noise.

2. Description of the Prior Art

When a signal such as the one shown in FIG. 1 is received under the influence of low-frequency noise, this signal may become distorted. The signal is, for example, used as a driving signal for a capacitive touch sensor. The capacitive touch sensor includes a plurality of detecting electrodes. When the driving signal is provided to one or more of the detecting electrodes, self-capacitive coupling by the detecting electrodes themselves, or mutual-capacitive coupling between the electrodes may be generated. When an external conductive object approaches or touches the capacitive touch screen, changes in capacitive coupling will occur in some of the detecting electrodes. Through this change in capacitive coupling, the location of the external conductive object can be determined.

However, low-frequency noise may easily inject into the capacitive touch sensor through the external conductive object, it may distort the change in capacitive coupling as just described, thereby creating errors in the determination of the location of the external conductive object.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the prior art, an input signal may be easily interfered by low-frequency noise. Referring to FIGS. 2 and 3, signal filtering circuits in accordance with a first embodiment of the present invention are shown. To address the noise interference issue described in the prior art, the present invention proposes that a plurality of cycles of the input signal is detected in each detection instance, and only the maximum and the minimum signals in each cycle are detected.

When the input signal is affected by low-frequency noise, the input signal is carried on the low-frequency signal, so the input signal will undulate with the low-frequency noise. Assuming that the difference between the amounts of interferences experienced by the first half and the second half of each cycle by the low-frequency noise is small, then the difference between the signal difference between the maximum and the minimum signals within the same cycle when the low-frequency noise is present and the signal difference between the maximum and the minimum signals within the same cycle when the low-frequency noise is not present will also be small. Moreover, detecting a plurality of cycles can reduce said difference.

In order to obtain the maximum and the minimum signals in a cycle, an example of the present invention is to employ the signal filtering circuits shown in FIGS. 2 and 3. An input signal Vin is provided to the circuits of FIGS. 2 and 3. In the positive half cycle, the input signal Vin will pass through a diode of the signal filtering circuit of FIG. 2, and is held by a capacitor until the maximum signal Vmax is held by the capacitor. As such, the maximum signal Vmax can be obtained by an output signal Vout of the signal filtering circuit of FIG. 2. Similarly, in the negative half cycle, the input signal Vin will pass through a diode of the signal filtering circuit of FIG. 3, and is held by a capacitor until the minimum signal Vmin is held by the capacitor. As such, the minimum signal Vmin can be obtained by an output signal Vout of the signal filtering circuit of FIG. 3.

However, diodes tend to have very large forward-bias limitation in integrated circuits. A typical forward bias is around 0.7 V. If the input signal is possibly less than or less than the forward bias, then it cannot be detected.

Accordingly, another example of the present invention employ signal filtering circuits shown in FIGS. 4 and 5. The signal filtering circuit may include a capacitor C, a switch S and a comparator Comp. An input signal Vin is inputted through the switch S and held by the capacitor C, and an output signal Vout is generated. The comparator Comp compares the input signal Vin and the output signal Vout to control the switch S.

Referring to FIG. 4, when the positive half cycle of the input signal Vin is fed into the signal filtering circuit of FIG. 4, Vin>Vout, the switch S is turned on, the capacitance of the capacitor C will keep increasing until it reaches the maximum signal, such that the output signal Vout equals to Vmax. Thereafter, Vin<Vout, the switch S is turned off, and the capacitance of the capacitor C will remain at the maximum signal until the whole half cycle is finished.

Similarly, referring to FIG. 5, when the negative half cycle of the input signal Vin is fed into the signal filtering circuit of FIG. 5, Vin<Vout, the switch S is turned on, the capacitance of the capacitor C will keep increasing until it reaches the minimum signal, such that the output signal Vout equals to Vmin. Thereafter, Vin>Vout, the switch S is turned off, and the capacitance of the capacitor C will remain at the minimum signal until the whole half cycle is finished.

In the above descriptions, the positive half cycles and the negative half cycles of the input signal Vin need not be provided to the signal filtering circuits of FIGS. 4 and 5, respectively, that is, the same input signal Vin can be provided to the signal filtering circuits of FIGS. 4 and 5 simultaneously, and the signal filtering circuits of FIGS. 4 and 5 can output the maximum signal and the minimum signal, respectively.

Moreover, the signal filtering circuits of FIGS. 4 and 5 need not be synchronized with the input signal. All that is required is to obtain the maximum and the minimum signals in each detection cycle according to the frequency (or the period) of the input signal, so that the signal differences between the maximum and the minimum signals in a plurality of cycles can then be obtained. One with ordinary skill in the art can appreciate that the signal filtering circuits of FIGS. 4 and 5 may also be synchronized with the input signal; the present invention is not limited to these. However, in contrast to the prior art where the input signal has to be synchronized with the detecting circuit of the input signal, the present invention has the advantage of requiring no such synchronization. Furthermore, according to the above, in a best mode of the present invention, the period of the input signal and the detection period of the signal filtering circuits are the same. In another example of the present invention, the period of the input signal and the detection period of the signal filtering circuits are not the same. For example, the detection period of the signal filtering circuits is smaller than the period of the input signal or the detection period of the signal filtering circuits is larger than the period of the input signal.

In an example of the present invention, the input signal can be sent by a pointing device, such as an active capacitive pen, which has a predetermined frequency, such as the frequency of the input signal. When the active capacitive pen is close to a capacitive touch sensor, the capacitive touch sensor will receive the input signal, and the signal is passed through the signal filtering circuits of FIGS. 4 and 5, so that the maximum and the minimum signals are outputted at each detection cycle of the signal filtering circuits, the detection of at least one cycle is accumulated for determining the location of the active capacitive pen. The capacitive touch sensor may include horizontal or vertical long strips of electrodes intersecting each other at a plurality of intersections. In an example of the present invention, the signal filtering circuits are coupled to each of the horizontal electrodes and each of the vertical electrodes, respectively. When the active capacitive pen is near some of the electrodes, these electrodes detect an input signal, which is used for determining the location of the capacitive pen. For example, the signal difference between the maximum and the minimum signals in at least one cycle of each electrode is detected, and the signal differences are summed together to produce a detecting signal. As such, a plurality of horizontal and vertical detecting signals can be obtained, so that the centroid locations of the detecting electrodes in the horizontal and the vertical directions can then be determined. As another example, change in the detecting signal of each electrode before and after the approach of the pointing device is first determined to obtain a plurality of changes in the vertical and horizontal detecting signals, and then the centroid locations of the detecting electrodes in the horizontal and the vertical directions can be determined. The detecting signals are produced by a detecting circuit receiving the output signals generated by the signal filtering circuits.

The capacitive touch sensor may detect the approach or touch of a hand and an active capacitive pen in a multiplexing way. For example, detection of the hand and the detection of the active capacitive pen are performed alternately. In an example of the present invention, when a capacitive pen is detected, the location of the hand is not provided to a host or an operating system, thereby achieving palm-rejection writing by pen, which allows a palm of a user can press against the capacitive touch sensor while writing using an active capacitive pen, for example. In another example of the present invention, the locations of the hand and the active capacitive pen can be both provided to the host or the operating system.

In an example of the present invention, a device providing the input signal (e.g. the above pointing device) may detect the working frequency of the capacitive touch sensor, for example, detect a signal sent from the capacitive touch sensor while the capacitive touch sensor is performing hand detection in order to determine the working frequency of the capacitive touch sensor, and to further adjust the frequency of the input signal to be the same or close to the working frequency of the capacitive touch sensor, wherein the detection period can be adjusted by the working frequency.

In another example of the present invention, the above detecting circuit may detect the frequency of the input signal, and based on the frequency of the input signal, further adjust the detection period of the signal filtering circuits or the working frequency of the capacitive touch sensor.

Since low-frequency noise may affect the capacitive touch sensor through a hand, the signal filtering circuits of the present invention can effectively reduce or eliminate the interference of the low-frequency noise, and has the advantage that the input signal does not have to be synchronized with the signal filtering circuits.

In the first embodiment, the signal filtering circuits are required to reduce or eliminate the interference of the low-frequency noise. The present invention also provides a method and circuit for reducing or eliminating the interference of the low-frequency noise without the use of the signal filtering circuits. Referring to FIG. 6, a circuit and method for detecting small signals in accordance with a second embodiment of the present invention is shown.

First, as shown in step 610, a plurality of signal values is generated based on an input signal. The signal values of the input signal can be first detected by a signal detecting circuit. For example, the input signal is passed through an integrator circuit for signal integration and then converted into digital signal values by an ADC (analog-to-digital converter). The signal detecting circuit may take multiple samples in one cycle to obtain a plurality of signal values. Each sample may generate one signal value through at least one integration, thus generating signal values S₀, S₁, S₂, . . . , S_(n), respectively, for example, as shown in FIG. 7.

Next, in step 620, a differential operation is performed on the signal values to create a plurality of difference values. Each difference value is the difference between a signal value and a preceding (or following) signal value, thus creating difference values D₀, D₁, D₂, . . . , D_(n), wherein D₀=S₀−S₁, D₁=S₁−S₂, D₂=S₂−S₃, . . . , D_(n)=S_(n)−S_(n+1). When the sample interval of the signal values is very small, the amounts of interferences of the low-frequency noise experienced by two adjacent signal values will be similar to each other. By subtracting the two adjacent signal values from one another, the interferences of the low-frequency noise can be substantially canceled.

Then, in step 630, a plurality of absolute values is generated based on the difference values, and the absolute values are summed to produce a detecting signal value.

In addition to suffering from the low-frequency noise, the input signal can also be affected by high-frequency noise interference. In order to address these two interferences at the same time, referring to FIG. 8, step 630 of FIG. 6 is replaced by step 640. In step 640, absolute values of a plurality of moving averages of the difference values are generated, and these absolute values are summed to produce a detecting signal value. The difference values produce moving averages Z₀, Z₁, Z₂, . . . , Z_(n), wherein

${Z_{n} = {\sum\limits_{i = n}^{n + k - 1}\; D_{i}}},$

and k>1, and the detecting signal value S=Σ|Z|=|Z₀|+|Z₁|+ . . . +|Z_(n)|.

As such, the high-frequency noise interference can be reduced through the moving average approach. The higher the value of k, the more the reduction of the high-frequency noise interference.

Referring now to FIG. 9, an active capacitive pen 9 in accordance with a third embodiment of the present invention is shown. The active capacitive pen 9 includes an internal coil 91, a rectifier 92, a DC converter 93, an AC signal generator 94 and a signal transmitting end 95.

The internal coil 91 can obtain an input signal through electromagnetic induction via an external coil 97, which serves as the power supply for the active capacitive pen. The input signal may be provided via the rectifier 92 (e.g. a bridge rectifier) to the DC converter 93 (e.g. a low dropout regulator) for providing a DC signal. The AC signal generator 94 (e.g. a “555 calculator”) provides an AC signal by using the DC signal as the power supply, and the AC signal is sent out through the signal transmitting end 95. In an example of the present invention, a voltage booster (such as a level shifter) can be added between the AC signal generator 94 and the signal transmitting end 95 to increase the electric potential of the AC signal.

In an example of the present invention, the external coil 97 can be provided in a capacitive touch sensor as described before. The external coil 97 may be provided in proximity to or around the electrodes. Alternatively, one or more electrodes act as the external coil 97. The present invention does not limit the form of the external coil 97. The external coil 97 can provide a signal for electromagnetic induction with the internal coil 91 by a controller. The controller can also include the detecting circuit as described before. In other words, the active capacitive pen 9 obtains external power through electromagnetic induction as the source of power for its internal circuits. One with ordinary skill in the art can appreciate that the active capacitive pen can also include a battery or a capacitor. External power is obtained through electromagnetic induction for charging the battery or the capacitor, and as the source of power for its internal circuits.

Thus, when the active capacitive pen 9 approaches or touches the capacitive touch sensor, power is obtained from the external coil 97, and an AC signal is sent from the signal transmitting end 95. In an example of the present invention, the external coil 97 may continuously provide power. In another example of the present invention, the external coil 97 may intermittently provide power. For example, power is provided during detection of a hand, while power is not provided during detection of a pen. Once the active capacitive pen 9 obtains power, it immediately sends out the AC signal. Since the change in signals injected by an active capacitive pen to the electrodes is opposite to the change in signals by a hand touching or approaching the electrodes, the pen will be not mistakenly determined as a touch or an approach of a hand. In an example of the present invention, if an approaching pen is detected during the detection of a hand, the hand is ignored, i.e. the coordinates of the location of the hand are not provided. During the detection of a pen, although the external coil 97 is not providing any power, the active capacitive pen 9 may still have some remaining power, and is still able to send out AC signals for a short time for capacitive coupling with the capacitive touch sensor 98, and thus injecting the input signals. Accordingly, the detecting circuit will be able to determine the location of the active capacitive pen 9.

The above provision of the location of a hand may include the provision of a plurality of locations approached or touched by hand(s). In addition, the detecting circuit may detect the AC signal (the input signal) sent by the active capacitive pen 9 according to the first or the second embodiment. Without the need for an internal power supply (e.g. a battery), the active capacitive pen 9 of the present invention can be made very thin and light, and no complicated circuit is required for synchronization with the detecting circuit. The present invention thus has the advantages of a simple structure and high applicability.

Moreover, the active capacitive pen may further include a tilt switch for detecting the degree of tilt of the active capacitive pen and turning on/off the power of the active capacitive pen based on the degree of tilt. For example, when the active capacitive pen is placed flat or close to horizontal, for example, at an angle of less than 30 degrees with the horizontal line, the power of the active capacitive pen is turned off. The power is provided by an internal battery of the active capacitive pen, for example, so when the active capacitive pen is placed flat or closed to horizontal, it will stop emitting any AC signal, or alternatively, the entire active capacitive pen is powered off to preserve power. The switch may include mercury, which is stored in a chamber within the pen. The two opposite ends of the chamber have the electrodes of the switch, respectively. When the active capacitive pen is placed flat or closed to horizontal, mercury inhibit conduction of the two electrodes, but when the active capacitive pen is tilted over a certain angle, mercury will conduct the two electrodes, allowing the active capacitive pen to emit AC signals, or allowing the entire active capacitive pen to be turned on. In addition, when power is coming from the outside, the above tilt switch can be used to stop the active capacitive pen from sending out the AC signals. Moreover, the above tilt switch can be a gravity sensor.

Furthermore, the above controller may include a counter, a counting circuit, or a program for counting how long a capacitive pen has left or moved away from the capacitive touch sensor. When the capacitive pen has not yet left or moved away from the capacitive touch sensor, or when the time the capacitive pen has not yet left or moved away from the capacitive touch sensor is less than a threshold, hand detection is stopped until the capacitive pen has left or moved away from the capacitive touch sensor for a period of time longer than the threshold.

This is done because, when writing on a capacitive touch sensor using a pen, the pen may temporarily leave the capacitive touch sensor, and during this time, a hand touching the capacitive touch sensor may be mistakenly regarded as a touching pen, causing confusion in the writing track of the pen. Thus, it is proposed that within a predetermined period of time since the pen moved away from the capacitive touch sensor (when the absence of the pen is less than the threshold), hand detection is not performed in order to avoid this problem.

FIG. 6

610 Generate a plurality of signal values based on an input signal 620 Perform a differential operation on the signal values to create a plurality of difference values, each difference value is the difference between a signal value and a preceding (or following) signal value 630 Generate a plurality of absolute values based on the difference values, and sum the absolute values to produce a detecting signal value

FIG. 8

610 Generate a plurality of signal values based on an input signal 620 Perform a differential operation on the signal values to create a plurality of difference values, each difference value is the difference between a signal value and a preceding (or following) signal value 640 Generate a plurality of absolute values of moving averages based on the difference values, and sum the absolute values to produce a detecting signal value 

What is claimed is:
 1. A writing detecting device for suppressing interference of low-frequency noise, comprising: a pointing device for providing a sine wave with a fixed period; a touch screen including a plurality of conductive strips composed of a plurality of vertical conductive strips and a plurality of horizontal conductive strips for receiving the sine wave through capacitively coupling; and a controller including: a detecting circuit for starting at least one signal detection at any arbitrary phase of the received sine wave, each signal detection lasting for a cycle, wherein the detecting circuit includes: a first signal filtering circuit for filtering out a maximum signal at each signal detection; a second signal filtering circuit for filtering out a minimum signal at each signal detection; and a signal difference generating circuit for generating a respective signal difference between the maximum and the minimum signals for each signal detection; and a summing circuit for summing all the signal differences for the at least one signal detection to generate a complete detected signal.
 2. The detecting device of claim 1, wherein the first signal filtering circuit includes: a first diode that only allows the positive cycle of the sine wave to pass through; and a first capacitor for holding the maximum signal provided by the first diode during each signal detection.
 3. The detecting device of claim 1, wherein the second signal filtering circuit includes: a second diode that only allows the negative cycle of the sine wave to pass through; and a second capacitor for holding the minimum signal provided by the second diode during each signal detection.
 4. The detecting device of claim 1, wherein the first signal filtering circuit includes: a first capacitor for holding the maximum signal received during each signal detection; a first comparator including a first negative input, a first positive input and a first output, wherein the first positive input receives the sine wave, and the first negative input receives the maximum signal held by the first capacitor; and a first switch for providing the sine wave to the first capacitor according to a signal of the first output when the signal at the first positive input is greater than the signal at the first negative input.
 5. The detecting device of claim 1, wherein the second signal filtering circuit includes: a second capacitor for holding the minimum signal received during each signal detection; a second comparator including a second negative input, a second positive input and a second output, wherein the second negative input receives the sine wave, and the second positive input receives the minimum signal held by the second capacitor; and a second switch for providing the sine wave to the second capacitor according to a signal of the second output when the signal at the second positive input is greater than the signal at the second negative input.
 6. The detecting device of claim 1, wherein the controller performs the at least one signal detection with an internal clock cycle, and the phase of the internal clock is asynchronous with the phase of the sine wave.
 7. The detecting device of claim 6, wherein the period of the internal clock and the period of the sine wave are the same.
 8. The detecting device of claim 7, wherein the controller adjusts the period of the internal clock based on the period of the sine wave.
 9. The detecting device of claim 7, wherein the controller further includes providing a driving signal to one conductive strip or a set of conductive strips based on the period of the sine wave, wherein the pointing device receives the driving signal via capacitively coupling or electromagnetically induction with the one conductive strip or the set of conductive strips being provided with the driving signal, and changes the period of the sine wave based on the period of the driving signal.
 10. A detecting method for suppressing interference of low-frequency noise, comprising: receiving a sine wave with a fixed period; starting at least one signal detection at any arbitrary phase of the received sine wave, each signal detection lasting for a cycle and including: detecting a maximum signal and a minimum signal; and calculating the signal difference between the maximum and the minimum signals; and summing all the signal differences for the at least one signal detection to generate a complete detected signal.
 11. The detecting method of claim 10, wherein the maximum signal is detected by a first signal filtering circuit, the first signal filtering circuit includes: a first diode that only allows the positive cycle of the sine wave to pass through; and a first capacitor for holding the maximum signal provided by the first diode during each signal detection.
 12. The detecting method of claim 10, wherein the minimum signal is detected by a second signal filtering circuit, the second signal filtering circuit includes: a second diode that only allows the negative cycle of the sine wave to pass through; and a second capacitor for holding the minimum signal provided by the second diode during each signal detection.
 13. The detecting method of claim 10, wherein the maximum signal is detected by a first signal filtering circuit, the first signal filtering circuit includes: a first capacitor for holding the maximum signal received during each signal detection; a first comparator including a first negative input, a first positive input and a first output, wherein the first positive input receives the sine wave, and the first negative input receives the maximum signal held by the first capacitor; and a first switch for providing the sine wave to the first capacitor according to a signal of the first output when the signal at the first positive input is greater than the signal at the first negative input.
 14. The detecting method of claim 10, wherein the minimum signal is detected by a second signal filtering circuit, the second signal filtering circuit includes: a second capacitor for holding the minimum signal received during each signal detection; a second comparator including a second negative input, a second positive input and a second output, wherein the second negative input receives the sine wave, and the second positive input receives the minimum signal held by the second capacitor; and a second switch for providing the sine wave to the second capacitor according to a signal of the second output when the signal at the second positive input is greater than the signal at the second negative input.
 15. A detecting device for suppressing interference of low-frequency noise, comprising: a receiving circuit for receiving a sine wave; a detecting circuit for starting at least one signal detection at any arbitrary phase of the received sine wave, each signal detection lasting for a cycle, wherein the detecting circuit includes: a first signal filtering circuit for filtering out a maximum signal at each signal detection; a second signal filtering circuit for filtering out a minimum signal at each signal detection; and a signal difference generating circuit for generating a respective signal difference between the maximum and the minimum signals for each signal detection; and a summing circuit for summing all the signal differences for the at least one signal detection to generate a complete detected signal.
 16. The detecting device of claim 15, wherein the first signal filtering circuit includes: a first diode that only allows the positive cycle of the sine wave to pass through; and a first capacitor for holding the maximum signal provided by the first diode during each signal detection.
 17. The detecting device of claim 15, wherein the second signal filtering circuit includes: a second diode that only allows the negative cycle of the sine wave to pass through; and a second capacitor for holding the minimum signal provided by the second diode during each signal detection.
 18. The detecting device of claim 15, wherein the first signal filtering circuit includes: a first capacitor for holding the maximum signal received during each signal detection; a first comparator including a first negative input, a first positive input and a first output, wherein the first positive input receives the sine wave, and the first negative input receives the maximum signal held by the first capacitor; and a first switch for providing the sine wave to the first capacitor according to a signal of the first output when the signal at the first positive input is greater than the signal at the first negative input.
 19. The detecting device of claim 15, wherein the second signal filtering circuit includes: a second capacitor for holding the minimum signal received during each signal detection; a second comparator including a second negative input, a second positive input and a second output, wherein the second negative input receives the sine wave, and the second positive input receives the minimum signal held by the second capacitor; and a second switch for providing the sine wave to the second capacitor according to a signal of the second output when the signal at the second positive input is greater than the signal at the second negative input. 